Increasing demands for high signal power and bandwidth capacity in modern communication networks impose stronger limitations on the allowed level of intermodulation distortion (IMD). A typical level of IMD is about −75 dBc for existing high power circulators, which is not sufficient for providing the desired degree of inter-channel isolation. The suppression of IMD decreases the interference between the adjacent communication channels and leads to the higher quality of operation. Therefore, the development of a circulator/isolator that is capable of handling high input power while maintaining a low IMD would be highly desirable.
A major contributor to IMD in microwave ferrite devices, such as circulators/isolators, is the non-linear phenomenon of ferromagnetic resonance. The closer the frequency of ferromagnetic resonance (FMR) is to the operating frequency range, the larger is the signal distortion.
Another contributor to the IMD is a non-uniform design. Specifically, the more portions of different conducting materials used in a device design, the worse the device performs in terms of IMD. For example, in surface mountable devices, separate conductors are typically used to electrically connect the center conductor to contact ports of the device. Moreover, it is difficult to provide tight coplanarity in such a design because the contact ports are distinct, separate parts from the connecting conductors. Such a non-uniform contact can contribute to increased IMD levels.
Another problem associated with the microwave ferrite devices is poor alignment. In more detail, typical circulators/isolators include a number of layers such as ferrites, a center conductor, magnets, pole pieces, ground plates, and temperature compensators. These layers are generally referred to as a stack. During manufacturing of a ferrite device, the layers are stacked onto one another, and manually manipulated by a technician during an alignment process before the layers are fixed into place. As the ground planes are generally the widest layers, it is difficult to properly align the narrower layers, such as the ferrites and center conductor. As such, alignment error is difficult to avoid.
Generally stated, the overall performance of the circulator/isolator device is a function of alignment. In addition, the shaping of the center conductor can be such that a low IMD level is achieved. The center conductor is usually shaped to match the circulator's impedance to that of a transmission line. Such impedance matching enables efficient transfer of energy between the device ports. The tuning elements typically include quarter-wave transformer arms and open-end tuning stub resonators symmetrically situated between the arms. With proper alignment, the open-end tuning stub resonators can fully extend to the perimeter of its surrounding layers, thereby enabling further improvement of IMD performance.
One common alignment technique employs a well in a housing, where the layers of a device can be stacked. The diameter of the well is slightly larger than the widest layer to accommodate the elements of the stack during manufacturing. The sides of the well are slotted allowing a manual push-stick alignment of the circuit. However, such a technique does not effectively solve the alignment error problem. In addition, thermal stress caused by differing coefficients of thermal expansion associated with the stack layers is further exacerbated by alignment error, thereby causing further deterioration of device performance.
The lack of coplanarity gives rise to other problems as well. In particular, large-scale production of ferrite devices implicates simple mechanical designs that are compatible with automated pick-and-place assembling and mounting technology. The proper pick-and-place of a device having a non-uniform, non-coplanar mounting base is inhibited. Thus, the manufacturing process may require more complicated and/or costly placement processes. Moreover, reliable electrical contact with a host system (e.g., a mother board or chassis level card) requires that the connecting leads and mounting base of a circulator/isolator be rigid and coplanar. Typically, an overall coplanarity of the mounting base should be within a few mils.
Thus, both electrical and mechanical parameters of a circulator/isolator device should be suitable for pick-and-place processing in both the device assembly, as well as population of the device on a host system. What is needed, therefore, is a highly manufacturable and reliable circulator/isolator device that has a co-planar mounting surface and is capable of maintaining a low IMD.